Peelable lids for canned goods, such as foods items or the like, are generally comprised of deformable steel or aluminum lids clinch double seam-sealed to a steel or aluminum can body. These lids are problematic and disadvantageous in many respects. For instance, they pose serious health hazards for children as they may cut themselves on the exposed edge of the peeled lid when licking the lid or the can flange. Difficulties are also encountered when manually breaking the seal and subsequently peeling the metallic lids off the can body. Further, the metallic lids, when separated from the can, are often discarded as waste rather than being recycled.
Recently, significant advances have been made in canning technology which have replaced the steel or aluminum peelable lids with thermoplastic peelable lids which are heat-sealed to metallic bodied cans. As shown in FIG. 1 and 2, these thermoplastic lids 15 include a peelable lid portion 16 premounted or preformed on a thermoplastic fusing ring portion 17 which is to be heat fused to a circumferential flange 18 at an open end 20 into a metallic can body 21. FIGS. 2A and 2B illustrate that fusing ring 17 includes an annular groove 22 formed for receipt of circumferential flange 18 therein (FIG. 2A). By applying the appropriate heat and pressure, to be described in greater detail below, fusing ring 17 is fused to can flange 18.
Once these thermoplastic lids 15 are fused to can body 21, these lids have proven easier to open and are safer to use by children than metallic lids in that the edges of the thermoplastic lid and flange may be licked without concern for injury. In addition, while these lids are completely recyclable, they do not pose as significant an environmental waste when discarded as disposed metallic goods. Typical of these heat-sealable thermoplastic lids may be found in U.S. Pat. Nos. 5,246,134 to Fraser et al.; and 5,125,528 to Heyn et al.
While these thermoplastic peelable lids are superior in form and function, as compared to metallic peelable lids, these arrangements are difficult to manufacture. Because lid 15 is deformable, to fuse ring portion 17 to can body flange 18, the can body 21 must be heated quickly to a sufficient temperature for localized contact melting of the lid. During the heating of the can body, a fusing pressure is continuously applied between the can and the thermoplastic lid to induce melting through conductive contact. Once the fusing ring portion is sufficiently melted, the fusing pressure is substantially increased to facilitate adhesion between the two fused components. Since this technique enhances adhesion, the burst pressure of the seal is also substantially increased.
Through capping machinery already known in the field, the thermoplastic lid can be automatically seated and oriented atop the can body for mounting engagement with flange 18. Subsequently, the can/lid combination is positioned to enter a lid sealing machine 23 for fusing of the lid to the can body. As shown in FIGS. 3 and 4, current thermoplastic lid sealing technology places the can/lid combination in an upright position which is movably supported on a first belt conveyor assembly 24. A second belt conveyor assembly 25 is positioned in opposed relation to first belt conveyor assembly 24 which cooperates therewith to apply a continuous fusing pressure between the can and the lid, and to drive the can/lid combination along a pathway formed between the first and second conveyor assemblies. The first and second conveyor assemblies 24, 25 include opposing drive belts 26, 27, respectively, which are positioned a fixed predetermined distance apart which squeeze the can and lid together to create the continuous fusing pressure as the can/lid combination passes therethrough.
To heat can body 21, the second conveyor assembly includes an induction heating coil 28 (FIG. 4) extending longitudinally along the pathway just above upper drive belt 27. This coil quickly heats the can body as it passes through a electromagnetic field created by coil 28 which induces a current through the can body causing it to heat up and melt the thermoplastic ring.
A set of shims 29 are positioned under first conveyor assembly belt 24 downstream from induction heating coil 28 which creates a pressure bump in the pathway. This increase in fusing pressure, substantially greater than the continuous fusing pressure applied between the opposing belts, enhances seal integrity which increases the can bursting pressure. As the can flange heats and melts the plastic in contact therewith, the fusing pressure pushing the lid and can together begins to decrease. This occurs because the distance between opposing belts 26, 27 is fixed, and as the plastic lid melts onto the flange, the can/lid height combination decreases. During continued melting of the plastic fusing ring, the lid begins to float on the liquid plastic. Insertion of shims 29 under the lower belt 26 forces can body 21 up against the lid as the belt carries the can through this shimmed region. The substantially increased pressure helps heat transfer from the can flange to the plastic and helps prevent the melted plastic from vaporizing and causing bubbles in the seal area. Subsequently, upon cooling, the cans are ready for filling and seaming of the metallic can bottom to the can body.
While this thermoplastic can sealing machine is adequate when all components of the machine are functioning properly, many .mechanical and sealing problems are associated with these first generation machines. Severe problems may occur when the can height varies by as little as 0.03 inches. Since the predetermined distances between the opposing belts are fixed, even at the pressure bump area, tall cans may penetrate the flange of the can too deeply into the thermoplastic lid which can deform or crush the can body. In contrast, too short of a can may not penetrate the flange deep enough into the thermoplastic fusing ring which adversely affects seal integrity. Additionally, tilting of the lids can occur when the fused lids are not constantly urged against can flange after passing through the pressure bump region during cooling and solidification of the thermoplastic lid to the can body.
Moreover, when this prior art sealing apparatus 23 is transporting, heating, fusing and curing a plurality of cans between the conveyors, simultaneously, one or both of the belt drives 26, 27 sometimes slips relative one another. This causes differences in the relative speed between lower belt 26 and upper belt 27 which, even with small speed differences when transported along the pathway, can cause the warm plastic lid to creep. This creep may deform the lid from its normal symmetrical shape, again potentially affecting seal integrity. Finally, malfunction of the capping machine may cause severe damage to the drive belts. As the can heats up during passage through the induction heating coils, contact of the heated can flange with the thin plastic belts can burn holes therethrough which ultimately requires frequent replacement of the belts.